These days, fabrication of an aspherical lens becomes popular for reducing aberration problems to make small optical systems. Hand-held optical systems such as camera phone, portable digital camera and camcorder accelerate the usage of the small optics and aspherical lenses. In spite of the demanding need for aspherical lenses, the aspherical lens is not widely used, since the process for fabricating an aspherical lens is a hard process until now. Apart from aspherical lenses, making non-spherical lens for example parabolic, cylindrical, or array of lenses, is also time consuming and difficult process.
Also fabricating a large lens gives another difficulty for lens makers. Fresnel type lens is a good solution for making large lenses without handling large and heavy materials. But the quality of the Fresnel lens is not that good as the conventional spherical lens. Fresnel lens offers only procedure reducing thickness. While making a large optics, aberration control is another serious problem other than fabrication itself. As the size of the lens becomes larger, the aberration of the lens system becomes severe. This is especially critical for the spherical lens system. The main reason for using the aspherical lens is to reduce the aberration of the optical system. Again the fabrication process of the aspherical lens is far more difficult than that of the spherical lens.
To overcome the difficulties in fabricating lenses, a new method for lens fabrication is introduced. Gradient index lens is a good example. Instead of geometrical variation, change of index of refraction gives the same effect as a lens. Using the gradient index of material and geometrical variation together, aberration of the system can be reduced. Although the gradient index lens gives significant reduction of the aberration, it is still expensive and hard to be fabricated.
In the present invention, the inventors provide a new method of lens fabrication introducing Micromirror Array Lens (MMAL). MMAL was invented for variable focal length lens and the properties of MMAL can be found in the U.S. Pat. No. 6,934,072 to Kim, U.S. Pat. No. 6,934,073 to Kim, U.S. Pat. No. 6,970,284 to Kim, U.S. Pat. No. 7,031,046 to Kim, U.S. patent application Ser. No. 10/857,714 filed May 28, 2004, U.S. Pat. No. 6,999,226 to Kim, U.S. patent application Ser. No. 10/893,039 filed Jul. 16, 2004, U.S. patent application Ser. No. 10/983,353 filed Mar. 4, 2005, and U.S. patent application Ser. No. 11/191,886 filed Jul. 28, 2005. While maintaining converge and phase condition of the MMAL, the MMAL can be fabricated to have a fixed focal length instead of variable focal lengths. The fixed focal length MMAL has lot of advantages and can solve the fabrication problems of the conventional lens.
First, the fabrication process of the MMAL is size independent. Since the MMAL is using standard semiconductor fabrication processes, making process of the MMAL is only dependent on the substrate wafer size. If the size of the lens is less than that of substrate wafer, then fabrication process is the same. Second, different kinds of lenses can be fabricated together. While fabricating the conventional lenses, the curvature of the lens determines the fabrication capability. Only one kind of lens can be fabricated together. While fabricating the MMAL, many different kinds of the MMALs can be fabricated together. Third, since the MMAL is an adaptive optical element, aberration of the system can be corrected by introducing the MMAL. Conventional lens has a severe problem due to aberration. Each micromirror can be designed to correct the problems of aberration of the optical system. Fourth, mass productivity is a major advantage of the MMAL. Since MMAL is fabricated by using standard semiconductor procedures, mass production of lenses can be easily achieved. Also since the MMAL is arranged in a flat surface, the MMAL reduces the size of the optical system and also critically reduces the problems of mounting optics. And last, the fixed focal length MMAL has a great advantage over variable focal length MMAL. Since the structure of the micromirror can be simplified, the fabrication becomes extremely simple. Also the production price is very cheap so that the fixed focal length MMAL can substitute the conventional lens in optical systems.
In the present invention, the fabrication process is made simple and the lens surface forming process is newly invented. By introducing the MMAL with surface profile shape memory, simple MMAL can be fabricated without loosing the great advantages of the MMAL.